1. Field of the Invention
The present invention relates in general to fiber pinning tools and in particular to a fiber pinning tool for structurally applying fiber stiffeners to a material.
2. Related Art
Many structural composites, such as structural composite airframes, usually consist of multiple stiffeners. The stiffeners supply rigidity and stiffness that is required under certain flight load conditions. One typical stiffener is a mechanical fastener, such as a hat stiffener. Hat stiffeners, named for their shape, are typically applied to aerospace structural composite components via their skin.
One process for fabricating hat stiffeners is to co-cure the hat to the skin of the structural composite material concurrently with the curing of the structural composite material itself. Another process for fabricating hat stiffeners is to secondarily mechanically bolt and/or adhesively bond the stiffener to the skin. However, in either case, the failure mode for both processes occurs at the inner hat to skin surface.
In order to resolve this problem, z-pinning is used. Z-pinning refers to applying reinforcing fibers in the same direction (or at a canted angle, such as 45.degree.) that the composite fibers of the structural composite material reside in. In contrast, the X and Y direction is in-plane with the fibers of the structural composite material. The stiffeners are usually pinned in the Z-direction to the skin with large pinning fibers, such as boron fibers. The pinning fibers are typically imbedded in a material, such as Teflon. This technique has been found to increase the mechanical properties of the stiffener to skin interface.
In one method (hereinafter referred to as "multiple fiber pressing"), the Z-pinning process typically consists of first softening the composite stiffener and skin with heat and then driving plural fibers through the skin using a press. Since numerous fibers are forced into the skin at once, pressure needed to insert the plural fibers covering large areas at once must be done hydraulically. In another method (hereinafter referred to as "individual fiber insertion"), the Z-pinning process typically consists of forcing individual fibers through the skin one at a time. The fibers are forced into the skin with an ultrasonic device, which uses high frequency for vibrating the fiber and eventually forcing it into the skin.
Both processes are lacking. For instance, the multiple fiber pressing method is very costly and expensive since custom tooling for the press is required to match and properly interface the specific pinning fibers with the composite material. In addition, it would be impossible to z-pin fibers in a 45.degree. angle at the inner hat to skin interface with this process. The individual fiber insertion method is very time consuming, tedious, requires many man hours (approximately 2 hours per in.sup.2 of inserted fibers), and is not suitable for a production process. As such, these pressing methods are inefficient and not ideal for large scale components and manufacturing tooling.
Therefore, what is needed is a fiber pinning tool for efficiently structurally applying fiber stiffeners to a material. What is also needed is an inexpensive process for efficiently applying fiber stiffeners to a material. Whatever the merits of the above mentioned systems and methods, they do not achieve the benefits of the present invention.